Any type of irradiation that displaces atoms can produce voids in non-fissile metals and alloys, consequent swelling and mechanical property effects becoming important in fast reactor design and operation. The basic reason for void formation is preferential absorption of interstitials at dislocations. Void growth depends on competition between alternative sinks for diffusing vacancies and interstitials. Irradiation and materials variables affect void formation through both the point defect behaviour and the dislocation behaviour. Void ordering and annealing effects are also discussed.